The invention is directed to an improved method and apparatus for the thermal treatment of fine-grained material, particularly for the manufacture of cement clinker.
More particularly, the invention relates to a system wherein the material is first pre-heated with hot exhaust gases in a pre-heating zone, the pre-heated material is heated further by supplying and burning waste materials such as used tires and is further calcined, and the material is subsequently completely calcined to form cement clinker in the sintering zone of a rotary tubular kiln.
It is known in the manufacture of cement clinker from raw cement meal to also introduce used tires into the admission chamber of the rotary tubular kiln into which the raw meal pre-heated in the pre-heater is introduced. Thermal energy with which the raw cement meal is partially calcined before being sintered is intended to be released by burning the used tires as waste fuel. Burning used tires in the rotary tubular kiln, however, can present difficulties in the overall cement clinker production line. The used tires intermittently thrown into the admission chamber of the rotary tubular kiln should burn in the rotary tubular kiln with oxygen that must be conducted past the flame of the rotary kiln burner arranged at the opposite end of the rotary kiln in an air excess. As a consequence of the non-uniform ignition and burn-out of the tires, these burn at different rates and sometimes also burn incompletely, so that a great formation of CO occurs at times in the exhaust gas of the rotary kiln. It has also been often observed that the CO surges are measurable even though oxygen is simultaneously present in the gas flow. This leads to the conclusion that the CO content in the exhaust gas that only break down slowly. These sometimes strong CO surges in the exhaust gas of the rotary kiln can be identified following the pre-heater as seen in the direction of gas flow and they must be avoided in view of the exhaust gas scrubbing, particularly when this is composed of an electrostatic dust separator, as well as for reasons of environmental protection. In order to suppress such CO surges in the exhaust gas of the rotary kiln, the air excess at the rotary kiln burner has hitherto been increased. The flame temperature of the rotary kiln burner, however, is thereby reduced and the quantity of rotary kiln exhaust gas as well as the specific heat requirement of the cement clinker production line increases.